Airway hyperresponsiveness (AHR) is a component of asthma, and the complex inheritance of both asthma and AHR have made it difficult to find the genetic etiology of these important problems. Analysis in a less heterogeneous genetic system than the human population could be useful for identifying causal loci. We and others have attempted this using quantitative trait locus (QTL) analysis in the mouse, which provides an excellent model for naTve AHR, but the results of these studies have been inconsistent. As part of this Program, we have used a very different approach to address this. We created phenotypically selected recombinant congenic mice to identify loci associated with increased naTve AHR. The seventh generation hyperresponsive mice retained A/J loci on chromosomes 2, 6, and 10. Surprisingly, analysis of unselected N8 progeny demonstrated that the naive AHR phenotype was not significantly associated with any of the loci individually, but was highly significantly associated with an interaction of loci on chromosomes 2 and 6. These findings were confirmed in an independent analysis of consomic mice. Also, as part of the Program, we generated A/J mice that are genetically depleted of mast cells. These mice do not show naive AHR, demonstrating that this trait is mediated by mast cells. The identification of genomic regions containing loci causally associated with AHR, the demonstration that this trait requires their interaction, and the observation that mast cells are required for expression of the disease phenotype has important implications for the dissection of the genetic etiology of asthma in humans. Of interest is that the protease ADAM33, which has been associated with the human disease by genetic analysis, is within the retained A/J region. An advantage of model systems is that they can facilitate functional analysis, and we propose to examine the role of ADAM33 using haplotype analysis and transgenesis. We also propose to further examine the role of mast cells in mediating the naive AHR observed in A/J mice by reciprocal bone marrow transplant and adoptive transfer. Lastly, we will continue to narrow the genetic interval in which the causal loci reside, with the aim of ultimately identifying the genes using a positional cloning strategy.